Threshold Braking is the maximum use of your cars available traction for deceleration. In practice Threshold Braking is instant and precise application of extremely hard pressure on the Brake Pedal to the point of impending wheel lock. Impending Wheel lock is when the tire is on the edge of sliding (You will hear a scrubbing - low squealing sound just before the wheel stops rotating) When done properly, Threshold Braking is the fastest way to stop on most surfaces. It is even results in slightly shorter stops than most Anti-Lock Brake systems can accomplish.
Most ABS systems allow the wheel to stop rotation, they then release pressure and re-apply. The time between pressure release and re-application is where the ABS system loses to a good driver who is using threshold braking. Newer high performance ABS systems allow better individual wheel control, and quicker apply / release cycles. Some ABS systems, like those on the Porsche 911 and the Chevrolet Corvette are sensitive and responsive enough to still allow the driver to use Threshold Braking and will only intervene in panic stops or on extremely slippery surfaces.
On extremely slick surfaces, or in panic stop or emergency maneuvering where the driver is caught by surprise, ABS has the inherent advantage of being a "No Brainer" (ie... It always works, even if you are too excited/panicked to use the proper techniques)
For Practice, you should use a wide unobstructed space, with lots of "Run Off" room.
accelerate the car to a reasonable speed and then try to stop the car as fast (Short) as you can, without any of your tires fully locking.